The present invention relates to the field of communication systems. In particular, the present invention relates to a method and system for mitigating interference in a communication system.
In communication systems, a large amount of information is exchanged between various hosts. A wireless communication system is one such system. In conventional wireless communication systems, multiple base stations provide a coverage area, with each base station being an access point covering one cell, i.e., a smaller region in the coverage area.
Multiple access techniques are utilized to enable simultaneous communication between a base station and multiple terminals. The terminals and base stations, of a portion of or all the cells, communicate by using the same Radio Frequency (RF), in order to increase spectrum efficiency. This is conventionally known as the frequency re-use mechanism.
When a large number of terminals communicate with multiple Base Transceiver Stations (BTS) at the same time, many different kinds of signal degradations take place. Apart from the omnipresent thermal noise, the degradations may be caused by either interference or channel-gain fluctuations. The lafter is conventionally known as channel fading.
In frequency re-use, a signal transmission in one cell is degraded by signals transmitting in neighboring cells, for uplink as well as downlink. This is known as inter-cell interference, which, along with thermal noise, is referred to as Other Interference (OI).
Mutual interference is generated at the receiver for both uplink and downlink communication due to channel degradations. Examples of channel degradations include non-orthogonal codes being used by different terminals; multi-path delay dispersion; imperfect timing, frequency synchronization, radio frequency circuitry and channel estimation. If multiple terminals communicate with a base station, each with a single channel, the mutual interference is called Multiple Access Interference (MAI). If a single terminal communicates with the base station, using multiple codes, the mutual interference is called Self-interference (SI). Both MAI and SI exist in wireless communication systems. These interferences are also referred to as intra-cell interference.
An important characteristic of intra-cell interference is that the power of the interference is a monotonically increasing function of the transmission power of the desired signal. Hence, at some point, increasing the signal transmission power may not improve the quality of the signal. Instead, increasing the signal transmission power may increase inter-cell interference in other cells.
Conventionally, a fast power-control mechanism is required to compensate for channel fading, based on the fast feedback of channel quality measurement. However, this consumes more bandwidth and may achieve high frequency power control only in Frequency Division Duplex (FDD) systems. For Time Division Duplex (TDD) systems, power control frequency is limited by the inverse of frame duration, and more efficient power control is needed.
Transmission rate and power control systems have been designed to overcome such limitations. One such power control system is described in U.S. Pat. No. 5,267,262, titled ‘Transmitter Power Control System’, assigned to Qualcomm Incorporated. In this patent, a combination of open- and closed-loop power control methods is described, to keep the power received by a receiver virtually constant. The receiver measures the signal power received and issues commands to a transmitter to increase or decrease it. However, the control is purely signal power-based, and is not distinguishable among different interferences and thermal noise.
Another power control method is described in U.S. Pat. No. 5,559,790, titled ‘Spread Spectrum Communication System and Transmission Power Control Method Therefor’, assigned to Hitachi Ltd., which monitors channel quality by using a pilot channel and an orthogonal noise observation code channel. However, this method may not accurately estimate either the signal power or the noise power, because of self-interference and/or multiple access interference. Further, the method described does not compute the self-interference.
Yet another power control method is described in US Patent application number 20040176033A1, titled ‘Wireless Communication System and Transmit Power and Data Rate Control Method Therefor’, assigned to Mattingly, Stanger & Malur, P. C. This patent application discloses a channel-coding rate for a multi-carrier modulation (OFDMA) system. The power control is purely power-based, and interference is not accurately tracked. The data rate control is utilized to adjust the channel-coding rate.
A data control method to maintain a target Packet Error Rate (PER) is described in US Patent Application number 20040179499A1, titled ‘Method and Apparatus for a Rate Control in a High Data Rate Communication System’, assigned to Qualcomm Incorporated. In this patent application, an open-loop algorithm measures the Signal-to-all-lnterference-plus-thermal-Noise Ratio (SINR) at regular intervals, and the close-loop algorithm measures the PER. The rate control scheme uses aggregate quality metric SINR or PER. However, no accurate channel information is explored and utilized.
When multiple terminals are communicating with a common base station, each terminal experiences different levels of inter-cell, intra-cell interference and channel gain fading. This phenomenon is called multi-user diversity. In such a scenario, the downlink medium is shared to maximize the throughput. The base station performs data rate control by scheduling transmission of only one terminal at any given slot when channel conditions are good for the terminal. However, this scheme introduces packet latency due to mismatch between traffic arrival and channel condition.
Therefore, to jointly address the aforementioned interferences, fading and multi-user diversity problems, and to provide a robust communication system, an efficient data rate and power assignment method is desirable. The method should incorporate accurate physical channel information regarding different types of interferences, fading, and the available transmission power.